<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>transitionend</title>
    <style>
        #box{
            width:100px;
            height:100px;
            background-color: blue;
            transition: width 2s ease;
        }
    </style>
</head>
<body>
    <div id="box"></div>
    <button id="start">start</button>
   
    <script>
       const box = document.getElementById('box');
       const start = document.getElementById('start');
       start.addEventListener('click',()=>{
        box.style.width = '200px';
       });
       box.addEventListener('transitionend',()=>{
        console.log('transition is end');
       });
    </script>
</body>
</html>